3-Aminomethyl-3,5,5-trimethylcyclohexaneamine (isophorone diamine, IPDA) can be prepared continuously by imination of 3-cyano-3,5,5-trimethylcyclohexanone (isophorone nitrile, IPN) with ammonia to produce a ketimine followed by catalytic hydrogenation. Disteldorf et al., U.S. Pat. No. 4,429,157, discloses the use of an organic ion exchange resin or zeolite as catalysts for the transformation of IPN to the ketimine using liquid ammonia. CA 2,309,328 discloses the use of metal oxide catalysts such as silica, alumina and titanias to perform a similar conversion to the ketimine.
To produce ketimine from a solution of IPN and ammonia requires sufficient residence time and high ammonia/IPN molar ratios to convert IPN to iminated product in a practical manner. For continuous operation of the imination step without catalyst, long reaction times of 120 to 180 minutes at 70.degree. C. and 650 psig, employing molar ratios of ammonia: IPN &gt;6:1 are required for high conversion to the ketimine. The ketimine, however, is not stable if kept in the presence of bases such as aqueous ammonia for long residence times. The ketimine decomposes by the elimination of HCN followed by reactions with IPN and ketimine. This leads to reduced yields of IPDA after the hydrogenation process. Although higher ammonia concentration and lower imination temperature can offset some of the yield losses, shorter imination times are desired to reduce by-product formation. If the imination residence time is reduced, however, unreacted IPN is hydrogenated in the second step to the corresponding amino alcohol product. DE 3,011,656, discloses a process to diminish by-product formation by carrying out the imination in a baffled reactor where backmixing of the fresh feed and reaction product is minimized even though there is longer residence time in the imination reactor. This results in an 85 mol % total IPDA selectivity in the hydrogenated product.
Heteropoly acids are a relatively new class of catalysts having redox properties and strong acidity. The heteropoly acids described here are limited to the Keggin-type. Their acidity is in the same range as that of sulfuric acid and sulfonic acids. U.S. Pat. No. 4,436,577 discloses soluble heteropoly acids used as catalysts for batch imination of 2-alkyl-4-amino-4-formylpyrimidine in methanol. The product aldimine was then catalytically reduced in a subsequent step with ammonia and base to 2-alkyl-5-aminopyrimidine. In this process, no separation or recovery of the catalyst was made, and recovery or recycling of the catalyst difficult, due to its solubility in the product and methanol. If large amount of catalyst is required the process becomes impractical.
CA 2,309,328 discloses the use of metal oxides, which can have both Lewis and Bronsted acidity, as imination catalysts for conversion of IPN to ketimine. Their acidity, however, does not approach that of sulfuric acid, sulfonic acids or strongly acidic ion exchange resins. The latter are not practically useful because of their cost and poor abrasion and thermal stability during the reaction. Metal oxides containing impregnated acid phosphates or sulfates are less stable in the presence of water.
There is a need for an acidic catalyst in the imination process described above which does not go into solution with the product formed; such a catalyst is provided herein by supporting a heteropoly acid on a refractory oxide or carbon such that it remains on the catalyst support during the imination step and provides the same catalytic acidity for the imination step as sulfuric and sulfonic acids without being removed during the imination process where water is produced as a product.